The radiographic image such as an X-ray image has been used over an extensive range in the field of medical treatment for the diagnosis of the state or progress of a disease. Specifically, the radiographic image based on an intensifying screen-film combination has been improved to enhance sensitivity and image quality in its long history. As a result, it is still employed in the field of medical treatment all over the world as an image pickup system characterized by both a high degree of reliability and excellent cost/performance ratio.
However, such image information pertains to so-called analog image information, which is not suited for free image processing or instantaneous transmission, unlike the digital image information that has been making a rapid progress in recent years.
In recent years, a radiographic image detecting apparatus of digital system represented by the Computed Radiography (CR) or flat panel detector (FPD) is coming on the market. The apparatus of this type directly provides a digital radiographic image, and directly shows an image on an image display apparatus such as a cathode ray tube or liquid crystal panel. It does not always require formation of an image on a photographic film. Thus, the X-ray image detecting apparatus of the digital system reduces the need of forming an image by silver halide photography, and hence provides a substantial improvement in the convenience of diagnostic operation in a hospital or clinic.
The Computed Radiography (CR) as one of the X-ray image digital techniques is currently being accepted in the field of medical treatment. However, it is insufficient in sharpness of the image and spatial resolution, and has not yet reached the image quality level of the screen/film system. A flat panel X-ray detector (FPD) using a thin film transistor (TFT) has been developed as a newer digital X-ray image technique, as disclosed, for example, in an “Amorphous Semiconductor Usher in Digital X-ray Imaging” by John Rowlands, “Physics Today”, November 1997, P. 24, or “Development of a High Resolution Active Matrix, Flat-Panel Imager with Enhanced Fill Factor” by L. E. Antonuque, SPIE, P. 2. Vol. 32.
A flat panel X-ray detector (FPD) has characteristics that the size of the apparatus is smaller than that of the CR and that the quality of the image obtained at a higher dose of X-ray is excellent, however, on the other hand, the SN ratio is lowered at a low dose of X-ray due to the electric noise generated by TFT or by the circuit itself, whereby the image quality is not fully enough.
A scintillator plate containing an X-ray phosphor which emits light when irradiated with X-rays is used to convert radiation into visible light. To improve the SN ratio in low-dose photographing, it is necessary to use a scintillator plate of high light emitting efficiency. Generally, the emitting efficiency of a scintillator plate is determined by the thickness of a phosphor layer and the X-ray absorbency coefficient of the phosphor. However, increase in the thickness of the phosphor layer results in scattering of the light emitted in the phosphor layer, whereby the sharpness of the image is reduced. Accordingly, the film thickness is determined when the sharpness required for image quality is determined.
Cesium iodide (CsI) is characterized by a relatively high conversion ratio of X-rays to the visible light, and is capable of easily forming a phosphor in a columnar crystal structure by evaporation. Thus, the scattering of the light emitted in the columnar crystal is reduced due to the light guiding effect, and the thickness of the phosphor layer can be increased (refer to Patent Document 1).
In a columnar crystal of the scintillator, emitted light by the scintillator repeats many times of reflection and absorption before the emitted light reach the light ejecting surface of the scintillator crystal. Half of the emitted light proceeds toward the leading end of the crystal where the light is reflected and a part of the reflected light proceeds to the light emitting plane which is in the inside of the crystal, however, the reflected light having a larger vector in the lateral direction forms a crosstalk component of light which enters into neighboring scintillator crystals or crystals existing faraway, whereby the image resolution has been lowered.
Patent Document 1: Japanese Patent Application Publication Open to Public Inspection (hereafter referred to as JP-A) No. 63-215987